As a photocatalytic material used in the application for stain-proofing and deodorization, titanium oxide is known. The photocatalytic material is used in various fields of interior and exterior building materials, home appliances such as lighting devices, refrigerators, air-conditioners, and toilets. However, titanium oxide cannot exhibit sufficient photocatalytic performance in indoor environments having only a small amount of ultraviolet rays because titanium oxide is excited by an ultraviolet region. Therefore, research and development have been in progress for a visible-light-responsive photocatalyst exhibiting photocatalytic performance even by visible light.
As the visible-light-responsive photocatalyst, tungsten oxide is known. A patent reference 1 describes a photocatalytic material made of tungsten oxide sputter-deposited on a base material, and tungsten oxide having a triclinic crystal structure is mainly used. Since the sputter deposition exposes the base material to high temperature, heat resistance temperature of some base material does not allow the application of the sputter deposition. Since the sputter deposition is often performed in a highly vacuum chamber, its process control is complicated, and it not only costs high depending on the shape and size of the base material but also has a difficulty in the deposition on a wide range such as on building materials. Moreover, though excellent in hydrophilic property, a visible-light-responsive photocatalytic layer made of sputter-deposited tungsten oxide has a problem that its performance of decomposing toxic gas such as acetaldehyde is not high enough.
The use of a tungsten oxide powder as a photocatalyst has been also studied. The tungsten oxide powder can be mixed with an organic binder to be applied on a base material, which eliminates the need to expose the base material to high temperature and makes it possible to form a coating film even on a wide range such as on building materials. As a method of manufacturing the tungsten oxide powder, there has been known a method of obtaining a tungsten trioxide (WO3) powder by heating ammonium paratungstate (APT) in the air (see a patent reference 2). The method of heating APT in the air provides a triclinic tungsten trioxide powder whose particle size is about 0.01 μm (BET specific surface area=82 m2/g).
The tungsten trioxide (WO3) powder generated by the heating of APT in the air needs to have fine particles in order to have improved photocatalytic performance. Applying a disintegration process can make the particle size small to some degree but has a difficulty in realizing the particle size of 100 nm or less, for instance. Moreover, turning it to fine powder by the use of the disintegration process causes a change in the crystal structure of the tungsten trioxide (WO3) fine powder due to a stress by the disintegration process. Since the stress of the disintegration process causes a defect of the occurrence of the re-combination of electrons and holes, it is thought that photocatalytic performance is deteriorated. The manufacturing method described in the patent reference 2 has a problem of low manufacturing efficiency of the tungsten trioxide powder since it requires 20 hour or more kneading in order to stabilize the BET specific surface area.
As a method of efficiently obtaining a fine powder, a patent reference 3, for instance, describes a thermal plasma process. A fine powder whose particle size is 1 to 200 nm is obtained by the application of the thermal plasma process. The thermal plasma process can efficiently provide a fine powder, but even if the tungsten oxide fine powder produced by the use of the method described in the patent reference 3 is used as a photocatalyst as it is, it is not always possible to obtain a sufficient photocatalytic property. It is thought that this is because the tungsten oxide fine powder produced by the thermal plasma method does not sometimes have an optimum optical property or crystal structure.
Tungsten oxide comes in various kinds such as WO3 (tungsten trioxide), WO2 (tungsten dioxide), WO, W2O3, W4O5, and W4O11. Among them, tungsten trioxide (WO3) is mainly used as a photocatalytic material because of its excellent photocatalytic performance and its stability in a room-temperature atmosphere. However, tungsten trioxide (WO3) has a disadvantage that its photocatalytic performance is not stable. Moreover, tungsten trioxide (WO3) cannot exhibit sufficient photocatalytic performance if its surface area is small.
Patent Reference 1: JP-A 2001-152130 (KOKAI)
Patent Reference 2: JP-A 2002-293544 (KOKAI)
Patent Reference 3: JP-A 2006-102737 (KOKAI)